Method for continuity logging

Abstract

A drill bit drilling a borehole is used as a strong high frequency, subsurface seismic source in combination with at least one receiver in a preexisting borehole to determine the continuity of a subsurface formation layer extending between the borehole being drilled by the drill bit and the borehole in which the receiver is located. The receiver(s) in the preexisting borehole may be any conventional seismic receiver or receivers, such as hydrophones or geophones. The receiver or receivers are positioned in the preexisting borehole at a depth adjacent the formation layer whose continuity is to be determined. The depth (position), velocity and thickness of the layer is determined from a conventional well log of the preexisting borehole. A frequency power spectrum for a preselected frequency range (the range is selected based on the logging data) may be computed from the signals recorded by the receiver(s). Several such spectra may be computed from signals recorded for several different time intervals and then summed to increase the signal-to-noise ratio. A large amplitude in this power spectrum (or the detected signals) is indicative of a low velocity layer that is continuous between the drill bit source and the borehole containing the receivers.

What is claimed is: 
     
       1. A method for determining the continuity of a preselected subsurface formation penetrated by a first borehole, comprising: locating at least one detector at a depth adjacent said preselected subsurface formation in said first borehole,   drilling a second borehole in the earth adjacent said first borehole,   detecting with said at least one detector acoustic energy generated by said drilling of said second borehole, and   determining the continuity of said preselected formation between said first and second boreholes from said detected acoustic energy.   
     
     
       2. A method as described in claim 1, further comprising: computing power spectra for a preselected frequency range from said detected acoustic energy, and   wherein said determining step employs said computed power spectra to detect amplitude increases in said preselected subsurface formation representative of continuity between said first and second borehole.   
     
     
       3. A method as described in claim 1, wherein said preselected subsurface formation is a low velocity layer. 
     
     
       4. A method as described in claim 3, wherein said low velocity layer is a porous or poorly consolidated formation. 
     
     
       5. A method as described in claim 1, further comprising: logging said first borehole and recording logging data.   
     
     
       6. A method as described in claim 5, further comprising: determining said preselected subsurface formation from said logging data.   
     
     
       7. A method as described in claim 6, further comprising: determining the thickness of said preselected formation layer from said logging data.   
     
     
       8. A method as described in claim 7, further comprising: determining the average seismic velocity of said preselected formation layer from said logging data.   
     
     
       9. A method as described in claim 8, further comprising: determining a preselected frequency range from said thickness and logging data.   
     
     
       10. A method as described in claim 9, further comprising: filtering said detected acoustic energy to exclude frequencies outside said preselected frequency range.   
     
     
       11. A method as described in claim 9, wherein said detecting step is for said preselected frequency range. 
     
     
       12. A method as described in claim 11, wherein said preselected frequency range is between a frequency determined by dividing said average seismic velocity by half the thickness of said formation layer and a frequency determined by dividing said average seismic velocity by one-tenth the thickness of said formation layer. 
     
     
       13. A method as described in claim 6, further comprising: determining a preselected frequency for said formation from said logging data.   
     
     
       14. A method as described in claim 1, wherein said detecting step comprises: detecting acoustic energy at a time when the depth of said second borehole is different than the depth of said preselected subsurface formation, and   detecting acoustic energy at a different time when the depth of said second borehole is approximately the depth of said preselected formation.   
     
     
       15. A method as described in claim 14, wherein said determining step comprises: comparing the detected acoustic energy from said times to determine amplitude increases at said different time relative to said other time.   
     
     
       16. Apparatus for determining the continuity of a preselected subsurface formation penetrated by a first borehole, comprising: means for detecting acoustic energy generated by a means for drilling a second borehole moveably locatable in said first borehole, and   means for determining the continuity of said preselected formation between said first and second borehole from said detected acoustic energy.   
     
     
       17. The apparatus of claim 16, further comprising: a band pass filter having a preselected frequency bandwidth for filtering acoustic energy from said means for detecting.